1. Field of the Invention
The present invention relates to a signal processor for processing signals which are reproduced in an information input/output apparatus for optically recording and reproducing information.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 57-58248 discloses a method for reducing crosstalk. In the disclosed method, three tracks which are adjacent to each other on an information recording medium are simultaneously irradiated with light beams. Signals from the tracks on both sides of the center track are multiplied by a predetermined coefficient and then subtracted from the signal from the center track. Accordingly the crosstalk can be reduced. However, the publication fails to mention how the coefficient is actually converged.
In Japanese Laid-Open Patent Publication No. 59-8104, a method of perturbation is shown as a method for adjusting how the signals from the tracks on both sides of the center track are subtracted from the signal from the center track. In the method of perturbation, a small amount of crosstalk is forcedly generated, and a phase of level variation of the crosstalk is detected, whereby an optimum point is obtained. However, in the method of perturbation, some slight amount of crosstalk inevitably remains in principle, and an oscillator is essentially required.
In Japanese Laid-Open Patent Publication No. 2-257474, a least-mean-square (LMS) error method is used for determining a coefficient considering the distortionless condition of the intersymbol interference in accordance with the employed modulation system. Specifically, a coefficient is determined by the LMS method so that the Nyquist's first criterion is satisfied.
In Japanese Laid-Open Patent Publication No. 2-281423, the error signal power is minimized by using a recording signal as a reference signal. However, according to this method, the recording signal to be recorded on the information recording medium is compared with a signal reproduced therefrom, so that it is impossible to reproduce information from a read-only information recording medium.
In Japanese Laid-Open Patent Publication No. 3-40225, the error signal power is minimized by comparing a preamble signal with a reference signal. In the case where the preamble signal is recorded on the information recording medium, the storage capacity of the information recording medium is reduced. Moreover, information cannot be reproduced from an information recording medium on which such a preamble signal is not recorded.
None of the above publications describes how a coefficient is determined so as to minimize the crosstalk occurring in an information recording medium. In addition, in the conventional case where a coefficient used in the crosstalk reducing device is constant, the crosstalk cannot be sufficiently reduced when information is to be reproduced from a recording medium having a small track pitch. Furthermore, in the conventional case where a coefficient used in an equalizer for reducing the intersymbol interference is constant, there exists no coefficient which can suppress the edge shift and can increase the noise margin.